This invention relates to cascade refrigeration systems wherein a first refrigeration circuit develops higher temperature refrigeration, which is provided to a refrigerant in a second refrigeration circuit, which then develops lower temperature refrigeration which is used to refrigerate a heat or refrigeration load such as is required in a food freezing operation.
The design and operation of virtually all cascade refrigeration systems pose an inherent optimization problem. In general, the evaporator temperature, Te, and load, Qe, on the low temperature or secondary circuit are known. The condensing temperature and ambient utility for the high temperature or primary circuit define the high side pressure of the primary refrigerant circuit. The intermediate operating temperature of the cascade condenser or refrigerant-refrigerant heat exchanger must subsequently be determined. Minimum system power consumption is achieved only when this intermediate temperature is appropriately identified. The subject optimization needs to be addressed at process design and actual operation. During actual process operation most systems may deviate substantially from the design load and conditions. In such situations the power consumption can be 5-10% higher than necessary. Most cascade control systems cannot readily extract this additional process efficiency. If and when online optimization is addressed, it is often through rudimentary techniques such as manual trial and error or simple heuristics.
Accordingly, it is an object of this invention to provide a method for operating a cascade refrigeration system which enables the provision of refrigeration to a heat load with reduced overall process power consumption than is possible with conventional cascade refrigeration system operation.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention one aspect of which is:
A method for operating a cascade refrigeration system comprising:
(A) compressing a first refrigerant in a first compressor, condensing the compressed first refrigerant, expanding the resulting first refrigerant to reduce the pressure and the temperature of the first refrigerant, passing the resulting first refrigerant to a heat exchanger, and vaporizing the resulting first refrigerant in the heat exchanger;
(B) compressing a second refrigerant in a second compressor, passing the compressed second refrigerant to the heat exchanger, condensing the second refrigerant in the heat exchanger by indirect heat exchange with said vaporizing first refrigerant, expanding the resulting second refrigerant to reduce the pressure and the temperature of the second refrigerant, and vaporizing the resulting second refrigerant by absorbing heat from a refrigeration load;
(C) monitoring the inlet and outlet pressure of each of the first compressor and the second compressor, monitoring the power consumption of each of the first compressor and the second compressor, and communicating the monitored pressure and power values to a process controller;
(D) operating the process controller to utilize the communicated pressure and power values to compute more efficient operating pressures for each side of the heat exchanger; and
(E) adjusting the operation of the first compressor and the second compressor to adjust the pressures of the first refrigerant and the second refrigerant being passed to the heat exchanger to be closer to the said more efficient operating pressures.
Another aspect of the invention is:
A method for operating a cascade refrigeration system comprising:
(A) compressing a first refrigerant in a first compressor, condensing the compressed first refrigerant, passing the condensed first refrigerant to a first receiver and thereafter expanding the first refrigerant to reduce the pressure and the temperature of the first refrigerant, passing the resulting first refrigerant to a heat exchanger, and vaporizing the resulting first refrigerant in the heat exchanger;
(B) compressing a second refrigerant in a second compressor, passing the compressed second refrigerant to the heat exchanger, condensing the second refrigerant in the heat exchanger by indirect heat exchange with said vaporizing first refrigerant, passing the condensed second refrigerant to a second receiver and thereafter expanding the second refrigerant to reduce the pressure and the temperature of the second refrigerant, and vaporizing the resulting second refrigerant by absorbing heat from a refrigeration load;
(C) monitoring the inlet and outlet pressure of each of the first compressor and the second compressor, monitoring the power consumption of each of the first compressor and the second compressor, and communicating the monitored pressure and power values to a process controller;
(D) operating the process controller to utilize the communicated pressure and power values to compute more efficient operating pressures for each side of the heat exchanger; and
(E) adjusting the quantity of the first refrigerant stored in the first receiver and adjusting the quantity of second refrigerant stored in the second receiver so that the operational pressures of the first compressor and the second compressor are closer to the said more efficient operating pressures.